1. Field of the Invention
This invention relates to a sheet transport apparatus for transporting a sheet-like member.
2. Description of the Related Art
Japanese Patent Laid-Open Publication Hei 3-102023 discloses a recording apparatus in which an auxiliary transport roller assembly is provided between a sheet feed section and a printing/transport section. The auxiliary transport roller assembly has no pinch roller and a sheet surface is brought into contact with an auxiliary transport roller surface by the resiliency of the sheet in a bent state whereby the recording sheet is transported.
However, if the sheet feed angle (to a horizontal plane) at which the sheet is transported from the sheet feed section to the printing section in this conventional apparatus is small, the resiliency force of the sheet produced by bending is reduced so that the sheet cannot be transported by the auxiliary transport roller assembly. The arrangement and construction of components of the apparatus are therefore restricted.
If a pinch roller facing the auxiliary transport roller assembly is provided to forcibly transport the sheet, an error is caused in the reverse movement of the sheet in accordance with a predetermined printing mode by the influence of the transporting force of the auxiliary transport roller assembly, resulting in occurrence of defects such as white or black lines in a printed image.
This printing mode will be described below with reference to FIG. 6.
In FIG. 6, a distance P represents a line feed pitch, a symbol .quadrature. represents a printing area, and a numeral in .quadrature. (a numeral shown next to .quadrature. in sentences of this description) represents printing order. In a case where enlarged characters I, J, K, and L are printed after printing ordinary size characters A, B, . . . G, and H , line .quadrature.1 is printed first, the sheet is then fed a distance P, and line .quadrature.2 is printed. Thereafter, the sheet is fed a distance P/2 in the reverse direction and line .quadrature.3 is then printed. If the value of P/2 is greater than the predetermined value, a white line is formed at the center of the enlarged characters, as shown in the letters J and K. If the value of P/2 is smaller than the predetermined value, a black line is formed at the center of the enlarged characters, as shown in the letters K and L. Some black line may be allowed but any white line is unallowable because it considerably reduce the quality of character prints.
That is, since the sheet movement in the normal direction is effected for a line change, the influence of the accuracy of the sheet movement upon the printed character area is small but the influence of the accuracy of the sheet movement in the reverse direction upon the printed character area is large.
Recording sheets used in the recording apparatus include ordinary paper, thick sheets or layers of paper, such as in postcards and envelops, and special sheets, such as thin plastic sheets.
Recording sheets are manually inserted or automatically supplied in such a manner that a plurality of recording sheets are set together and transported one after another by an automatic sheet feeder.
Recording sheets are guided by a guide member, which limits the number of sheets, to be set inside a separating claw without contacting a feed roller.
The above-described recording apparatus is ordinarily arranged in such a manner that recording sheets are supplied by being separated one after another by the rotation of the feed roller of the automatic sheet feeder, and, after recording, are stacked in a sheet discharge section by a sheet discharge roller provided downstream of a transport roller. An automatic feeder arranged to eliminate a back tension of the feed roller has also been proposed in which the feed roller has a semicircular cross section such as to avoid contact with a recording sheet.
In the conventional automatic sheet feeder, however, recording sheets may contact the semicircular feed roller when brought close to the feed roller to be set, because there is a variation in the initial angular position of the feed roller or because of instability of recording sheets. In the worst case, the feed roller rotates by following the recording sheets by a frictional force, so that the recording sheets cannot be set smoothly. There is also a possibility of a sheet feeding failure if the position of the feed roller is incorrect.
To avoid such problems, the extent of overlapping between the feed roller and the sheet number limiting guide member may be increased. In this case, however, the number of set sheets is necessarily reduced or the capacity of the sheet container is necessarily increased.